The problem is that since we don't vectorise the standard library yet, the vectoriser chokes on any calls to standard library functions. The desugarer introduces such calls (mostly to patError) even when they aren't necessary so we rely on gentle simplification to remove them before vectorising. This is indeed quite fragile but is only a temporary measure until we can vectorise the libraries.

I'll try to find out what exactly is happening there over the weekend.

I fixed the problems related to my library changes. Two tests are still failing (albeit for different reasons). They are dph-diophantine-fast and dph-words-fast. The former produces a wrong result and the latter triggers an assertion in vectors slice function. To me, this looks like functional bugs in the library.

Ben, could you maybe have a look at this, or maybe get George to check the involved primitives.